A characteristic of wireless communication is that a signal is received from multipaths. In other words, a signal arrives at a reception point through a plurality of transmission lines in the form of a direct wave, a reflected wave, a transmitted wave, a diffracted wave and the like. As these waves have different signal intensities, phases, and reception levels, fading occurs. In mobile communication, which is communication between the base station and the mobile station, the fading also occurs when a direct wave is shut off because of an obstacle, such as a building, or when the distance between the base station and the mobile station changes.
One approach to avoid the fading is to use the technique of site diversity in a mobile station. The site diversity technique includes receiving signals from a plurality of base stations, demodulating those signals and combining those signals. As a result, even if signal form one base station becomes weak, quality of communications with the other base stations can be maintained.
Another approach to eliminate the effect of the fading is to use CDMA, as a multiple access, separate a multipath fading wave from the signals by RAKE reception and combine the signals while making phases of the signals uniform.
The technique of the site diversity is disclosed in Japanese Patent Application Laid-Open Nos. H10-247873 and 2000-4215.
Japanese Patent Application Laid-Open No. H10-247873 discloses a technology in which base stations receive an error-correction coded signal from a mobile station, the base stations transmit the error-correction coded signals to a host apparatus, and the host apparatus carries out error-correction code coding, combining, and, error-correction code decoding of the error-correction coded signal. This technology makes it possible to improve the error rate of the received signal without increasing the amount of signal data flowing between the base stations and the host apparatus.
The Japanese Patent Application Laid-Open No. 2000-4215 discloses a technology in which an identical information data are subjected to punctured convolutional coding using mutually different puncturing patterns to obtain data punctured differently, and the data obtained is time-diversity transmitted as diversity branches. A reception unit that receives the punctured data conducts depuncturing the data by using the puncturing patterns that were used at the transmission side, combines the depunctured data, and subjects the combined data to convolution-decoding.
In the technology disclosed in the Japanese Patent Application Laid-Open No. H10-247873, all the base stations send an identical signal to the mobile station. Since the base stations send the identical signal, the quality of those signals, when they are received at the mobile station, vary greatly according to the state of a transmission line between the mobile station and each of the base stations. If the base stations are configured to performs error correction, for example, puncturing using a convolutional code or a turbo code for error correction, of the signal, the higher is the rate of puncturing, the more the signal is influenced by the state of the transmission line.
The technology disclosed in the Japanese Patent Application Laid-Open No. 2000-4215 performs same modulation irrespective of the state of the transmission line. As the number of modulation levels of the signals transmitted from the base stations is larger, the phases of these signals are close to each other so that the mobile station cannot satisfactorily demodulate the received signals. As a result, the site diversity can not be achieved sufficiently.